The body of a mail message consists of everything following the first blank
line to the end of the file.
When you sent your sendstuff
file, it contained only a body.
Now edit the file sendstuff and add a small header.

Subject: a test add add
This is a one line message.

The Subject: header line is an optional one. The sendmail
program passes it through as is. Here, the Subject: line is followed
by a blank line and then the message text, forming a header and a body.
Note that a blank line must be truly blank. If you put space or tab characters
in it, thus forming an "empty-looking" line, the header will not be
separated from the body as intended.

Send this file to yourself again, running sendmail by hand
as you did before:

% /usr/lib/sendmail you <sendstuff

Notice that our Subject: header line was carried
through without change: